1. Field of the Invention
The present invention relates to a semiconductor device for driving a light emitting element used in high speed optical communication.
2. Description of the Related Art
As demand for information transmission increases, the need for high speed/large capacity optical communication information transmitting systems increases. In the optical communication art, light emitting diodes (LEDs) and semiconductor lasers (LDs) have been used for converting electric signals to optical signals in transmitter devices.
As the amount of transmitted information increases, the on/off periods of the LEDs become smaller. When these on/off periods are reduced, the pulse widths of the light output signals differ from the pulse widths of the original electric signals due to various characteristics of the related electric circuits and the LEDs. Accordingly, it is difficult for the receiver to receive data signals which accurately represent the electronic data signals.
To overcome this problem, a circuit as shown in FIG. 2 was developed. Data signals for driving a light emitting element 25, which arrive at input terminal 20, are applied to two delay circuits 21 and 22. Delay circuits 21 and 22 each delay the respective signals for a different amount of time. These signals having different delay times are output from the delay circuits 21 and 22 and are logically combined by a logic circuit 23. Signals indicative of the result produced by logic circuit 23 are supplied to a drive circuit 24 which drives a light emitting element 25.
An SCFL (source coupled FET logic) invertor circuit is frequently used for each of the delay circuits 21 and 22. An example of this type of delay circuit 34 is shown in FIG. 3. In this circuit, the delay time is controlled by adjusting a gate voltage (control voltage V.sub.cnt), which is applied to constant current source 32 in source follower circuit 31 through input terminal 33. The control voltage V.sub.cnt can be fixed for the delay circuit 21.
This circuit arrangement has several problems. For example, when input data signals are delayed using an electrical delay circuit, the rising/falling time of the pulse of these signals increases with an increase of the delay time. Therefore, fluctuation of the input data signal pattern increases.